1. Field of the Invention
The present invention relates to an optical pickup, and more particularly, to an optical pickup detecting spherical aberration caused by thickness deviation of a recording medium, and/or compensating for spherical aberration caused by the thickness variation of a recording medium.
2. Description of the Related Art
In general, information recording/reproduction density increases as a size of a light spot focused on a recording medium in an optical pickup apparatus becomes smaller. The shorter a wavelength (λ) of light used and the larger a numerical aperture (NA) of an objective lens, the smaller the size of a light spot, which is expressed by equation (1):size of light spot α λ/NA  (1)
To reduce the size of the light spot focused on the recording medium in order to obtain a higher recording density, there is a need to construct an optical pickup with a short wavelength light source, such as a blue semiconductor laser, and an objective lens having a larger NA. A format for increasing recording capacity up to 22.5 GB with a 0.85-NA objective lens, and for reducing the thickness of a recording medium to 0.1 mm is desired so as to prevent degradation of performance caused by tilting of the recording medium. Here, the thickness of the recording medium is defined as a distance from a light incident surface of the recording medium to an information recording surface.
As shown in equation (2) below a spherical aberration W40d is proportional to a fourth power of the NA of the objective lens and to a deviation of the thickness of the recording medium. For this reason, if an objective lens with a high NA of about 0.85 is adopted, the recording medium must have a uniform-thickness with a deviation less than ±3 μm. However, it is very difficult to manufacture the recording medium within the above thickness deviation range.                               W                      40            ⁢            d                          =                                                            n                2                            -              1                                      8              ⁢                              n                3                                              ⁢                                    (              NA              )                        4                    ⁢          Δ          ⁢                                          ⁢          d                                    (        2        )            
FIG. 1 is a graph showing a relation between thickness deviation of the recording medium and wavefront aberration (optical path difference (OPD)) caused by a thickness deviation when a 400-nm light source and an objective lens having an NA of 0.85 are used. As shown in FIG. 1, the wavefront aberration increases proportionally with the thickness deviation. Thus, when the objective lens having a high NA, for example, an NA of 0.85, is adopted, there is a need to correct for spherical aberration caused by the thickness deviation of the recording medium.
FIG. 2 shows a conventional optical pickup detecting and correcting aberration, which is disclosed in Japanese Patent Publication No. hei 12-155979. Referring to FIG. 2, the conventional optical pickup includes a light source 10, an objective lens 17, which focuses a light beam emitted from the light source 10 onto a recording medium 1, and a half mirror 11 altering a traveling path of the light beam passed through the objective lens 11 after being reflected from the recording medium 1. A hologram optical element (HOE) 20 divides and deflects an incident light beam from the half mirror 11 into a light beam passing through a particular region and a light beam passing through another region. A photodetector unit 21 includes first through fourth photodetectors 21a, 21b, 21c, and 21d, which detect the light beam passed through the particular region (See FIG. 4). A signal processing circuit 23 detects aberration from the detection signals of the first through fourth photodetectors, and a wavefront changing device 25 changes the shape of a wavefront of the light beam going toward the recording medium 1 from the light beam source 10 according to a signal from the signal processing circuit 23. In FIG. 2, a collimating lens 13 collimates the light beam emitted and diverging from the light source 10.
FIG. 3 illustrates wavefront aberration resulting from spherical aberration. When spherical aberration occurs, retarded wavefronts 27a and 27b, which are symmetrical around an optical axis c, are generated with respect to a reference wavefront 27 at the aperture center. When spherical aberration occurs, leading wavefronts, which are symmetrically around the optical axis c, may be generated.
As shown in FIG. 4, the HOE 20 includes first and second diffraction areas 20a and 20b which select, divide and diffract a retarded wavefront portion such that divided light beam portions are symmetrical with respect to an x-axis crossing an optical axis and go toward the first and fourth photodetectors 21a and 21d. The HOE 20 also includes a third diffraction area 20c, which diffracts the light beam portion excluding the retarded wavefront portion above the x-axis such that a diffracted light beam portion goes toward the second photodetector 21b. A transmission area 20d transmits the light beam portion below the x-axis such that a transmitted light beam portion goes toward the third photodetector 21c. The first and second diffraction areas 20a and 20b have a semicircular shape.
Each of the first and fourth photodetectors 21a and 21d has a 2-sectional configuration with which the occurrence of spherical aberration can be detected by detecting the focus status. Each of the second and third photodetectors 21b and 21c has a 2-sectional configuration with which a focus error signal can be detected using a knife edge method.
FIGS. 5A through 5C illustrate the variations of light beam patterns received by the first through fourth photodetectors 21a, 21b, 21c, and 21d according to occurrence of wavefront aberration. In particular, FIG. 5A illustrates light beam patterns received by the first through fourth photodetectors 21a, 21b, 21c, and 21d when a retarded wavefront occurs. Retarded wavefront portions, which are diffracted by the first and second diffraction areas 20a and 20b of the HOE 20, are focused behind the first and fourth photodetectors 21a and 21d. The light beam patterns received by the first and fourth photodetectors 21a and 21d are symmetrical. Relatively higher amplitude signals are detected by a first section A of the first photodetector 21a and a second section D of the fourth photodetector 21d, compared with a second section B of the first photodetector 21a and a first section C of the fourth photodetector 21d. FIG. 5B illustrates light beam patterns received by the first through fourth photodetectors 21a, 21b, 21c, and 21d when no aberration occurs. As shown in FIG. 5B, the first and second sections A and B of the first photodetector 21a detect signals having the same magnitude. Also, the first and second sections C and D of the fourth photodetector 21d detect light signals having the same amplitude. FIG. 5C illustrates the light beam patterns received by the first through fourth photodetectors 21a through 21d when a leading wavefront occurs. In this case, the leading wavefront portions, which are diffracted by the first and second diffraction areas 20a and 20b, are focused before the first and fourth photodetectors 21a and 21d. Relatively higher amplitude signals are detected by the second section B of the first photodetector 21a and the first section C of the fourth photodetector 21d, compared to the first section A of the first photodetector 21a and the second section D of the fourth photodetector 21d. 
Thus, a spherical aberration signal SES′ is detected by subtracting a sum of a detection signal b of the second section B of the first photodetector 21a and a detection signal c of the first section C of the fourth photodetector 21d, from a sum of a detection signal a of the first section A of the first photodetector 21a and a detection signal d of the second section D of the fourth photodetector 21d, which is expressed as:SES′=(a+d)−(b+c)  (3)
If this conventional aberration detection method is applied, both an amount and a polarity of aberration can be detected with respect to a small amount of spherical aberration. Meanwhile, when a large amount of spherical aberration occurs due to saturation of the signal difference, only the polarity of the spherical aberration, not the amount thereof, can be detected.
Another drawback of the conventional aberration detection method lies in that predetermined amplitude of spherical aberration signal SES′ is detected even when only a predetermined amount of defocus occurs without spherical aberration. Defocus W20 is proportional to the square of an NA of an objective lens, which is expressed as formula (4). Thus, a the degree of retarding and leading in wavefronts caused by defocus and spherical aberration differs, but the characteristics of the retarded and leading wavefronts caused by defocus and spherical aberration are very similar.                               W          20                =                              1            2                    ⁢          Δ          ⁢                                          ⁢                      zNA            2                                              (        4        )            where Δz is the amount of movement of an image point.